(1) Field of the Invention
The present invention generally relates to a filter device, and more particularly to a filter device appropriate for use in a dual-band radio system which transmits and receives signals in dual bands.
Small-size, light-weight mobile terminal systems such as automotive telephones or portable telephones have come into widespread use. The number of subscribers of such systems has increased in recent years, and an increase in the number of channels which can be utilized by the mobile terminal systems is required. To meet the requirement, mobile terminal systems which use a wider range of radio frequencies than before have been developed for practical use. In particular, an earlier mobile terminal system utilized a frequency band of around 800 MHz only, and a new mobile terminal system utilizing a frequency band of around 1.5 GHz is in practical use.
More specifically, the earlier mobile terminal system utilizes a transmitting frequency range of 940-960 MHz and a receiving frequency range of 810-830 MHz. The new 1.5-GHz system utilizes a transmitting frequency range of 1.429-1.453 GHz and a receiving frequency range of 1.477-1.501 GHz.
(2) Description of the Related Art
FIG. 1 shows a radio signal portion of a prior art radio system. This system transmits or receives signals within a single frequency band of 800 MHz or 1.5 GHz. Hereinafter, such a mobile terminal system will be called a single-band system.
Referring to FIG. 1, the radio signal portion of the radio system includes an antenna 11, an antenna duplexer 2, a radio signal transmitter (TX) unit 3, and a radio signal receiver (RX) unit 4. In the single-band system, shown in FIG. 1, a modulator unit (not shown) is connected to an input of the TX unit 3, and an output of the RX unit 4 is connected to a demodulator unit (not shown).
The radio signal transmitter (TX) unit 3 receives a transmitting signal from the modulator unit at a filter 5. The filter 5 passes through only signals within a center-frequency band. A power amplifier (PA) 6 amplifies the transmitting signal from the filter 5. A filter 7 passes through only signals within the center-frequency band. The TX unit 3 outputs the transmitting signal from the filter 7 to the antenna duplexer 2 for transmitting the same via the antenna 11.
The radio signal receiver (RX) unit 4 receives a receiving signal from the antenna duplexer 2 at a filter 8. The filter 8 passes through only signals within the center-frequency band. A low-noise amplifier (LNA) 9 amplifies the receiving signal from the filter 8. A filter 10 passes through only signals within the center-frequency band. The RX unit 4 outputs the receiving signal from the filter 10 to the demodulator unit.
The filters 5, 7, 8 and 10 are usually band-pass filters which block all signals but those within a center-frequency band including a predetermined center frequency "fo". The filters 5, 7, 8 and 10 reject noise while passing through only the signals within the center-frequency band.
The single-band system for the 800-MHz band and the single-band system for the 1.5-GHz band, both of which have a construction similar to that of the above-described mobile terminal system, have been separately developed and put into practical use. The 800-MHz system cannot make use of signals within the 1.5-GHz band, and the 1.5-GHz system cannot make use of signals within the 800-MHz band.
A dual-band radio system which can utilize both 800-MHz and 1.5-GHz bands is useful for users. Such a dual-band radio system can make use of signals within one of the 800-MHz and 1.5-GHz bands, the one of the two bands being manually selected. In order to build the above dual-band radio system, some elements may be used in common for both the 800-MHz band and the 1.5-GHz band. However, other elements cannot be shared and must be separately prepared for the dual-band radio system.
For example, in the case of the radio system in FIG. 1, the power amplifier 6 and the low-noise amplifier 9 can be used in common for the above dual-band radio system. However, with respect to the filters 5, 7, 8 and 10, a set of filters for the 800-MHz band and a set of filters for the 1.5-GHz band must be separately prepared to build the above dual-band radio system.
The filters 5, 7, 8 and 10 are packaged parts which are separately prepared. Therefore, in order to build a dual-band radio system, a set of filters for one of the two bands and a set of filters for the other band must be prepared and incorporated. This makes it difficult to construct a dual-band radio system having a small-size, light-weight signal portion. In addition, the two sets of the filters must be assembled to construct the radio signal portion of the radio system, and reliability of the radio signal portion after the assembly becomes low and the cost becomes high. Further, it is difficult to obtain a desired characteristic of band-pass filtering by simply combining the two sets of the filters.